I've had such a long history xDDD
Kinda late but better late than never (?)
First I was Emerald Sceptile, because Emerald was my first game and Treecko was my starter, and Sceptile quickly became one of my all time faves. Plus Shiny Sceptile looks emerald-green in colour (or so I believed at the time) and it fit.
Later I took the name Triceratops Rex, simply because I liked both of them dinosaurs. I thought the horns were cool, as was the Rex bit. Not sure how many folks saw it as a combo of two dinosaurs, but that was my name for a while.
Then it became a bunch of things over the years (I've forgotten most), but I recall being ~Infinity at one point, simply because my name means "Infinity" in my language, and I thought ( still think so, fight me) tildes were (are) cool. Anyway, there were others called Infinity and I felt like a change, so I moved on.
Then, for sake of convenience (mine, others') I returned to my old username but dropped the Triceratops and became ~Rex (told you tildes were cool). This I believe is my longest held username, and subsequently, to some of the older users I'm Rex. The only reason I didn't change this before was because I kept missing PM's deadlines Dx
Finally, I settled on Quokka Koala, my two fave marsupials, as well as a pun on Coca Cola. Eventually Eppy and the others convinced me that it wasn't a) as rad and b) as clever as I thought as it was, and c) not many people knew what quokkas are anyway. So then I jumped to Empty Boxes which is a reference to a light novel I had read years ago, and had fallen in love with. I really wanted to have a username showcase my fondness for the light novels, and also have something unique that only other people in the fandom would understand.
Oh, and for a couple of days, I was Firefly Season 2, but decided it was too traumatizing a name to look at every time I used the site. [Cries]
Ok, that was longer than required, but thanks for the trip down the memory lane :)